It is known that an .alpha.-olefin can be polymerized by using a catalyst composed of a solid catalytic ingredient in which a titanium tetrahalide is carried on a magnesium compound and another component which consists of an organic aluminium compound. With respect to the above-mentioned polymerization, various approaches have been attempted to provide a new method which is effective for increasing the yield of the resultant polymer per unit weight of the solid catalytic ingredient in the catalyst used, to such an extent that the resultant polymer contains a very small content of the used catalyst therein and, therefore, an operation for removing the used catalyst from the resultant polymer can be omitted.
However, the attempted approaches were not always satisfactory. That is, some of the attempted approaches were not effective for increasing the yield of the resultant polymer to such an extent that the operation for removing the used catalyst from the resultant polymer could be completely omitted.
Recently, it was found that when a polyolefin produced by using the above-mentioned type of catalyst was directly used in a process for producing a shaped polymer article, without removing the residual catalyst from the polymer, the shaping apparatus, for example, a mold, was corroded by a halogen contained in the residual catalyst in the polymer. This phenomenon is now a large problem in the polyolefin industry. For example, in Polymer Journal, 19, 597.about.602 (1980), it is pointed out that the corrosion of the mold in the molding procedure of the polyolefin cannot be prevented unless the content of halogen in the .alpha.-olefin polymer is 30 ppm or less; in other words, the yield of the .alpha.-olefin polymer should be approximately 33,000 g or more per gram of the halogen atoms in the solid catalytic ingredient in the catalyst used.
Japanese Patent Application Laid-open Nos. 53-30681 (1978) and 55-75409 (1980) disclose a method for producing a polyolefin in an increased yield thereof, respectively. In this method, a feed containing an .alpha.-olefin is brought into contact with a catalyst composed of a specific solid catalytic ingredient and an organic aluminium compound to preliminarily polymerize the .alpha.-olefin, and, then, the feed is subjected to a final polymerization in the presence of the above-mentioned catalyst and the preliminary polymerization product. However, the yield of the resultant poly .alpha.-olefin obtained in the above-mentioned method is unsatisfactory. For example, according to Example 1 of Japanese Patent Application Laid-open No. 55-75409, the yield of the resultant poly .alpha.-olefin is only approximately 5,300 g per gram of the solid catalytic ingredient per hour of the polymerizing time. This yield is still unsatisfactory and, for the purpose of preventing the undesirable corrosion of the mold, it is indispensable to remove the residual catalyst from the .alpha.-olefin polymer used for the shaping procedure.
Previously, the inventors of the present invention provided an invention relating to a method for polymerizing an .alpha.-olefin at a large yield of the resultant .alpha.-olefin polymer per gram of the solid catalytic ingredient, especially, per gram of halogen atoms in the solid catalytic ingredient. This method is disclosed in Japanese Patent Application Laid-open Nos. 56-45909 (1981) and 56-163102 (1981).
However, it is still strongly desired to provide a new method effective for producing an .alpha.-olefin polymer at a larger yield thereof per gram of the solid catalytic ingredient used, more especially, per gram of halogen atoms in the solid catalytic ingredient used, than that of conventional methods, which new method allows the operation for removing the residual catalyst from the resultant polymer to be completely omitted.